The manufacturing process of flat panel display substrates requires specific sized glass substrates capable of being processed in standard production equipment. The sizing techniques typically employ a mechanical scoring and breaking process in which a diamond or carbide scoring wheel is dragged across the glass surface to mechanically score the glass sheet, after which the glass sheet is bent along this score line to break the glass sheet, thereby forming a break edge. Such mechanical scoring and breaking techniques commonly result in lateral cracks about 100 to 150 microns long, which emanate from the score wheel cutting line. These lateral cracks decrease the strength of the glass sheet and are thus removed by grinding the sharp edges of the glass sheet. The sharp edges of the glass sheet are ground by a metal grinding wheel having a radiused groove on its outer periphery, with diamond particles embedded in the radiused groove. By orienting the glass sheet against the radiused groove, and by moving the glass sheet against this radiused groove and rotating the diamond wheel at a high RPM (revolutions per minute), a radius is literally ground into the edge of the glass sheet. However, such grinding methods involve removal of about 100 to 200 microns or more of the glass edge. Consequently, the mechanical scoring step followed with the diamond wheel grinding step creates an enormous amount of debris and particles.
In addition, in spite of repeated washing steps, particles generated during edge finishing continue to be a problem. For example, in some cases particle counts from the edges of glass sheets prior to shipping were actually lower than subsequent particle counts taken after shipping. This is because the grinding of the glass sheets resulted in chips, checks, and subsurface fractures along the edges of the ground surfaces, all of which serve as receptacles for particles. These particles subsequently would break loose at a later time, causing contamination, scratches, and sometimes act as a break source in later processing. Consequently, such ground surfaces are "active", meaning subject to expelling particles with environmental factors, such as, temperature and humidity. The present invention relates to methods for reducing these "lateral cracks" and "micro-checking" caused by grinding, thereby forming a glass sheet having edges that are more "inactive".
Laser scoring techniques can greatly reduce lateral cracking caused by conventional mechanical scoring. Previously, such laser scoring methods were thought to be too slow and not suitable for production manufacturing finishing lines. However, recent advances have potentially enabled the use of such methods in production glass finishing applications. Laser scoring typically starts with a mechanical check placed at the edge of the glass. A laser with a shaped output beam is then run over the check and along a path on the glass surface causing an expansion on the glass surface, followed by a coolant quench to put the surface in tension, thereby thermally propagating a crack across the glass in the path of travel of the laser. Such heating is a localized surface phenomenon. The coolant directed behind the laser causes a controlled splitting. Stress equilibrium in the glass arrests the depth of the crack from going all the way through, thereby resulting in a "score-like" continuous crack, absent of lateral cracking. Such laser scoring techniques are described, for example, in U.S. Pat. Nos. 5,622,540 and 5,776,220 which are hereby incorporated by reference.
Unfortunately, unbeveled edges formed by laser scoring are not as durable as beveled edges, due to the sharp edges produced during the laser scoring process. Thus, the sharp edges still have to be ground or polished as described herein above. An alternative process has been to grind the edges with a polishing wheel made from a soft material, such as, a polymer, in order to smooth out the flat sharp edges formed by the scoring process. However, the polishing process often gives rise to a phenomenon that is known in the industry as an "edge roll", where during the finishing of an edge having a flat surface, the surface tends to roll over and form an associated radius.
In light of the foregoing, it is desirable to design a process to finish an edge of a glass sheet that curbs prospective chips, checks and subsurface fractures along the edge. Also, it is desirable to provide a process that allows a smaller amount of glass removal and yet maintain the edge quality. Furthermore, it is desirable to design a process that increases the speed of finishing an edge of a glass without degrading the desired strength and edge quality attributes of the glass. Also, it is desirable to provide a technique that provides an edge without blended radiuses.